A TETRAMERIC MODEL OF THE DIHYDROLIPOAMIDE DEHYDROGENASE COMPONENT OFTHE PYRUVATE-DEHYDROGENASE COMPLEX - CONSTRUCTION AND EVALUATION BY MOLECULAR MODELING TECHNIQUES

On the basis of the homodimeric X-ray structure of dihydrolipoamide de hydrogenase from Azotobacter vinelandii we demonstrate by protein mode ling techniques that two dimeric units of this enzyme can associate to a tetrameric structure with intense contacts between the building blo cks. Complementary structures of the respective other unit in the tetr amer contribute to the active sites. The coenzyme FAD becomes shielded from the environment, thus its binding is stabilized. By energy minim ization techniques binding energies and RMS-values were computed and t he contact areas between the building blocks were determined to quanti fy the interaction. In the cell tetramerization of dihydrolipoamide de hydrogenase will be realized upon its incorporation as an enzyme compo nent into the pyruvate dehydrogenase multienzyme complex and will have consequences for the structure and subunit stoichiometry of the compl ex. Especially, the multiplicity of the three enzyme components, i.e. pyruvate dehydrogenase, dihydrolipoamide acetyltransferase and dihydro lipoamide dehydrogenase in the enzyme complex must be 24:24:24 instead of 24:24:12 assumed so far.